Redshift

Introduction

• Redshift is a phenomenon observed in the spectra of light from distant celestial objects, such as stars and galaxies, and is a fundamental concept in astronomy and cosmology. It indicates the motion of these objects away from the observer.

Understanding Redshift

• Redshift refers to the shift in frequency and wavelength of light from distant celestial objects compared to what we observe from objects at rest. This shift is caused by the relative motion between the source of light and the observer.
• Redshift is analogous to the Doppler effect observed in sound waves, where the pitch of a sound changes as a source approaches or moves away from an observer.

Observation and Significance

• When astronomers compared the spectra of light from distant stars and galaxies with the spectrum of light from our Sun, they noticed a consistent pattern: the light from distant objects had a slightly decreased frequency and slightly increased wavelength. This indicated that these celestial objects were moving away from Earth, leading to a redshift in their spectra.
• This redshift is characterised by a shift towards the red end of the electromagnetic spectrum, meaning a lower frequency and longer wavelength.

Redshift Factor (z)

• The term "redshift" also refers to the factor z, which quantifies the relative change in wavelength due to the Doppler shift for a receding celestial object. It is a crucial parameter used in cosmology.

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Redshift

Calculating Redshift

• The Doppler equation used for sound calculations cannot be applied to galaxies moving at such high speeds, so a different equation is used for redshift calculations:
• z = (λobserved - λemitted) / λemitted
• Where:
  • z is the redshift factor.
  • λobserved is the observed wavelength of light.
  • λemitted is the wavelength of light emitted by the celestial object.

Example Calculation

• Suppose a distant galaxy emits a characteristic spectral line at 91 nm (ultraviolet light at the 'Lyman limit'). However, when observed on Earth, it appears to have a wavelength of 640 nm (red light). We can calculate the redshift using the redshift equation:
• z = (640 nm - 91 nm) / 91 nm
• After calculation, the redshift factor z can be determined.

Conclusion

• Redshift is a fundamental concept in astronomy and cosmology that helps astronomers understand the motion of celestial objects in the universe. It indicates that most galaxies are moving away from us, providing key evidence for the expansion of the universe. Redshift is quantified by the redshift factor z, and its calculation allows us to study the dynamics of the cosmos.

Summary

• Redshift is a phenomenon where the spectra of light from distant celestial objects show a shift towards longer wavelengths and lower frequencies, indicating their motion away from Earth.
• It is quantified by the redshift factor z, calculated using the difference between observed and emitted wavelengths.
• Redshift is a crucial concept in cosmology, providing evidence for the expansion of the universe and helping us understand the dynamics of the cosmos.